1. Field of the Invention
The invention relates to improvements in shielding for blocking propagation of electromagnetic interference ("EMI") associated with circuits, housings and subassemblies, of a type using a flexible electrically conductive sheet material that loosely encloses a source of EMI or a circuit susceptible to EMI, and thereby prevents high frequency energy from passing a shielding boundary.
According to certain inventive aspects, the flexible sheet material can be combined with or formed to define a gasket that is conductively sealable to an opposed conductive element such as a housing or backplane panel. This is useful to continue a conductively sealed enclosure at the perimeters of openings, for example, among other possibilities, around input/output connectors which traverse a shielding boundary.
The sheet material can have an electrically conductive surface exposed on one of two opposite faces of the sheet material, the other face being insulated such that contact with the shielded circuit does not produce an electrical short. Preferably, however, selectively conductive and insulated areas are provided on both sides of the sheet, for achieving certain objects. For example, a positive connection can be made by an exposed conductive surface on at least one side of the sheet material, for contact with one or more of the electronic equipment, a shielded circuit therein and a connector traversing a shielding boundary, typically for connecting to a circuit ground.
The sheet material can comprise one or more openings for access through a conductive area, for example at which the sheet material comprises a conductive flap overlying a hole in the sheet material. In the event of such openings, an insulating lip can be provided surrounding the opening. The conductive material can be spaced back from the edges of the opening, or covered over by an insulating surface material adjacent the edges, the insulating lip preventing electrical shorting adjacent the opening.
The sheet is suitable for the shielding of various types of electrical circuits including, but not limited to printed circuit cards of portable computers, notebook or palmtop devices, disc drives, power supplies and internal circuit subassemblies, and similar compact electronic devices for which signal or power conductors pass through the shielding boundary. The invention permits a nearly continuous and effectively gapless enclosure to be made and conveniently used for shielding relatively high frequencies associated with modern electronic equipment.
2. Prior Art
It is known to enclose electric circuits in a conductive enclosure coupled to a circuit ground, to attenuate electromagnetic radiation emitted from or received by the circuits. A conductive shielding enclosure may have gaps, which may or may not adversely affect the shielding efficiency or the extent of attenuation, depending on the frequencies to be shielded. To attenuate relatively high frequency radiation effectively, any gaps must be correspondingly small.
In connection with computing and radio communications equipment, the frequencies of clock oscillators, multiplying phase locked loops and similar circuits may be quite high. The basic clock speed of a conventional personal computer, for example, typically ranges from 25 to 100 MHz, and advances are pushing clock speeds upwardly. Circuits producing signals at different frequencies produce harmonics at the sum and difference of the frequencies. Data and clock signals comprising square waves are also characterized by harmonics. A computer or the like thus may produce objectionable harmonics up to 900 MHz and higher, requiring a very intensive shield, i.e., a nearly gapless enclosure which is quite conductive and is placed close to the circuits that emit or are sensitive to the electromagnetic radiation.
The most common forms of shielding comprise one or both of sheet metal structures forming boxes, and conductive plastic materials or coatings. A normally-nonconductive plastic can be made conductive by including metal fibers or particles, by applying a metallic coating layer, or by laminating the housing from a series of conductive and nonconductive sheets. Such techniques are disclosed, for example, in U.S. Pat. Nos. 5,137,782--Adriaensen, et al. (embedded wires); 5,164,542--Hart (laminated wire screen); 5,170,009--Kadokura (electrodeposited coating); and 5,226,210--Koskenmaki, et al. (conductive paint coating). Such techniques provide a form of shielding, but add to manufacturing complexity, particularly if the internal surface facing the shielded circuitry must be made nonconductive to avoid shorting, free of gaps, for example caused by scratching a coating, and provided with sufficient conductive and/or magnetically permeable material to substantially attenuate incident fields.
Although the most common shield material is sheet metal, shields can be made using metallized plastics. A product known as QuietShield, available from AMP, Inc., employs laminates of metalized plastic fabric and selectively applied insulating layers and adhesives, enabling the material to be folded into box-like shapes. In general, such products benefit from the low weight of the plastic materials and the capability of folding them.
Another possibility is to apply an insulating plastic coating directly on a circuit, component, or metal shield barrier. In U.S. Pat. No. 4,670,347--Lasik et al. insulating materials are laminated with a metal ink layer. In U.S. Pat. No. 5,166,864--Chitwood et al., or Japanese Patent Application H 2-77276--Kuno et al. (laid open Dec. 6, 1991), insulated shielding materials are attached directly onto a circuit card populated with components. This places the conductive material over the components, but is substantially permanent.
According to commonly owned U.S. patent application Ser. No. 08/168,939, filed Dec. 12, 1993, now U.S. Pat. No. 5,436,803, shielding can be advantageously provided in the form of a flexible nonconductive envelope with embedded conductive material such as sheet metal or conductive fibers sufficient to provide a low resistivity as needed for electromagnetic shielding, namely on the order of 10.sup.-1 to 10.sup.-2 .OMEGA./.quadrature. or less, and a nonconductive surface facing toward the shielded circuit. The envelope forms a loose flexible shielding bag and is arranged around the shielded circuit. An opening such as a shielded neck leading to an opening can be provided for input/output conductors which traverse the boundary of the envelope. The envelope is grounded, for example, to a ground on the electronic equipment contained within the envelope, or to a ground point on an input/output connector, etc.
It is known to provide electrostatic discharge protection as a permanent feature of a circuit card mounting, for example as in U.S. Pat. No. 5,005,106--Kiku, where such a structure protects the circuitry of an integrated circuit bank card or smartcard from electrostatic discharge, and resides in a plastic housing with the circuitry. This unit is self contained, and thus the electrostatic discharge material does not interfere with other aspects of circuit operation and mounting. The discharge-protective structure does not form a complete envelope around the affected circuit. Kiku uses a minimally conductive film rather than highly conductive embedded material as needed for shielding against electromagnetic interference. No means are provided such as an extension neck, access opening or the like for dealing with passage of conductors or access to the circuit elements.
It is known to provide an opening in an RF shield through which conductors can pass. U.S. Pat. No. 3,383,455--Kerley discloses an RF shield in the form of an elongated sleeve constructed of a metal mesh which encapsulates an electrical component to be shielded. A small opening is provided in the sleeve which is fitted with an insulating grommet, the opening providing an egress for the lead wires which originate from the component. The grommet appears to provide abrasion protection for the lead wires in the event mechanical vibration causes movement between the lead wires and the sleeve. Since the lead wires are isolated from the sleeve by the insulating grommet, the sleeve is not in any way electrically coupled to the lead wires. The grommet has a circular opening and several round conductors are passed through that opening, leaving gaps between the lead wires and the grommet. This type of arrangement is not structured to provide high attenuation in high frequency shielding applications.
U.S. Pat. No. 4,785,136--Mollet et al. discloses an EMI shielding cover for a computer terminal. The shielding cover is formed from a conductive fabric sheet. The conductive sheet has an opening with re-closable upper and lower tabs which provide a means for passing one or more cables through the sheet. Mollet is consistent with Kerley in that the conductive sheet is not in any way coupled to the ground or shield layer of the cable.
Circuits may be sensitive due to high gain, operation at high frequencies, the use of components and structures conducive to inductive or capacitive coupling, etc. Some of the same aspects may characterize circuits that emit offending high frequency electromagnetic radiation. To attenuate an incident EMI field in such situations requires substantially more conductive material arranged in a more complete conductive enclosure than may be needed, for example, to protect against damage from electrostatic discharge (ESD).
In view of the operational frequencies and character of many modem circuits, a reasonably adequate shield for EMI purposes would attenuate radiation at least by 50 to 60 dB over a frequency range of 30 MHz to 1.0 GHz or more. To achieve this the shield needs to be substantially continuous and must enclose the shielded circuits closely and completely, i.e., forming a closed highly conductive envelope. Whereas electrostatic discharge packaging may be sufficiently conductive to dissipate a static charge at a surface resistivity of 10.sup.+1 to 10.sup.+12 .OMEGA./.quadrature., electromagnetic shielding for attenuating incident EMI fields has a surface resistivity of 10.sup.-1 to 10.sup.-7 .OMEGA./.quadrature..
It would be advantageous to provide a flexible sheet which is formed at least partly into a flexible bag-like enclosure and is as highly conductive as a shield of rigid sheet metal panels and/or boxes. It would also be advantageous to provide a gasket which provides a conductive seal for input/output connectors which traverse the boundary of the enclosure. The present invention concerns a low-resistivity flexible shielding sheet that is formed at least partially into an enclosure that can be mounted in a device such as a portable computer or the like for shielding an included circuit assembly, or perhaps for shielding subassemblies forming part of the circuit assembly. The sheet can have conductive and insulated areas on both sides. At least one conductive area can provide a means for sinking EMI energy from the sheet to ground. At least one opening can be provided in a conductive area of the sheet, the opening providing access to the circuit and/or means for passing signal lines or power lines through the shield barrier. The conductive material is spaced back from the edges of the opening thereby defining an insulated edge which prevents shorting of electrical components on the circuit and/or other sensitive surrounding areas.